So some specific reactions to what everyone said here so I can unpack a bit more of my own feelings.
Regarding the theme of that one’s choices don’t matter, it’s very much a contrast to Undertale and how even the littlest things can change how things go. A single death ruins everything, after all. I definitely think it’s a deliberate theme, but I also don’t think it will be entirely truthful. I don’t distrust Toby’s words that there will be One Ending, and even if that’s the case it doesn’t mean things won’t affect other parts of the world.
Regarding Susie, she is definitely a great avatar for certain kinds of game players. People who are sick of reading dialogue, people who want to mash A and kill everything and move on. Her development was a teeny bit hamfisted in the later portions, I wanted her to more or less stay herself without making it obvious that she’s learned something from the journey. But overall she was done quite well.
Regarding the final ending, it is indeed the same no matter what you do, reinforcing the prior theme. The red heart represents us, and Kris for whatever reason does not seem to want us to control them. The cage is also worn and beaten if you check it before the ending, indicating that this kind of thing has happened before, which raises plenty of questions.
Regarding the Dark World’s own ending, I would say that the entire first chapter and game within the context of the Dark World is basically an Undertale microcosm. You get characters that draw directly on the influences of others, particularly Lancer feeling like Sans and Papyrus put together. And that’s probably why that version of the ending varies depending on whether you’ve beaten up anyone before then or not. Even letting Susie accidentally fight one person will turn the ending into that “rushed” version. However, interestingly, whether you endure the King’s attacks or fight back doesn’t matter in and of itself.
Regarding its connection to Undertale, I would say that it’s simply one of the many different timelines that exist in the grand scheme of things. I’m not sure what makes things different this time - monsters living alongside humans more readily, and on the surface with civilization as well. I’m sure this is something that would be explored more, but right now it leaves plenty of strange questions yet to be answered. There’s a lot I wish I knew, and I would love to just sit down and read Toby’s story outline even if the game at large isn’t finished ever… but I have faith that he will find a way to make it happen.
Regarding Ralsei… yep, he’s the best, and there’s still plenty of mystery surrounding him actually being Asriel - does that mean that Darkeners are monsters, or is he just an exception? I think the lyrics of the song at the end are Asriel assuring Kris that even though he might be far away and distant in the real world, he’ll always be with him “in the dark”, that is to say the Dark World.
Whew that was a lot of words! Sorry! I’m glad to get it out of my system though.